1. Field of the Invention
This invention generally relates to the field of materials construction and, more specifically, to an apparatus and method for constructing a composite structure.
2. Description of the Prior Art
Composite structures are desirable in many industries for many applications. For example, aircraft, space, and land/sea vehicles employ a variety of curved and multiple-contoured surface structures in their fabrication. Composite materials are commonly used for these structures because, among other desirable attributes, composite materials have high strength-to-weight ratios. Even so, composite structures formed from composite materials still need to be stiffened in some instances. Therefore, manufacturers of composite structures are continually searching for better and more economical ways of stiffening composite structures.
There are various systems and methods of stiffening composite structures. For example, in an aircraft context, hat and blade stiffeners are sometimes utilized. However, a problem with hat and blade stiffeners is that they can have special peel problems at their ends. Furthermore, when utilizing hat and blade stiffeners for strength in perpendicular directions, it becomes very expensive from a manufacturing standpoint because hat and blade stiffeners do not lend themselves well to criss-cross patterns.
Another common stiffening method is the use of honeycomb structures. However, honeycomb structures are usually manufactured as a sandwich structure, which means that they can trap moisture within them, leading to degradation, and are limited in depth because of volume and weight considerations. In addition, manufacturing honeycomb structures can be very expensive.
An additional method of stiffening composite structures is the use of waffle structures. Waffle stiffening can achieve the depths of hat and blade stiffeners, without the peeling problems, and can achieve bi-directional strength by nature of their criss-cross patterns. In addition, waffle stiffeners, unlike honeycomb structures, consume less volume and can be made to not trap moisture or fuel, which can be detrimental in aircraft applications. However, waffle stiffening is not without its problems. Prior methods of constructing waffle stiffeners result in sub-standard waffle stiffeners because of folding or kinking problems during lay-up of the composite plies. In addition, prior waffle stiffeners are limited in depth because of strength considerations, which means that the span between supports cannot be extended or increased, which would save a considerable amount of money during manufacture.
An apparatus and method for stiffening a panel provides a composite waffle stiffener and the method for constructing the stiffener. A plurality of fabric plies pre-cut to a selected pattern are laid on a tool having a waffle configuration, the plies overlapping each other to maintain continuity of the structure. The resulting waffle stiffener is then bonded to a panel requiring stiffening. The stiffener is flexible and maybe bonded to curved panels. Holes in the stiffener allow access to the volume between the stiffener and the panel for minimizing volume loss within, for example, a wing structure containing fuel and for ventilating air or moisture trapped in the volume.